Semiconductor devices communicate with their environment of use by accepting electrical impulses supplied by an external source (such as a circuit board) and conducting these impulses to electrical circuits contained on a semiconductor chip. The semiconductor chip reacts to the input in a predetermined manner to generate output. The input and output of electrical impulses to the semiconductor device occur over multiple paths of electrically conducting material, generally referred to as leads.
FIG. 1a shows one edge of a traditional semiconductor die 10 having a row 12 of individual bond pads 14 regularly spaced and arrayed at the periphery of one edge of the semiconductor die 10. A pattern of individual leads 16 is present. Individual bond wires 18 connect a single lead 16 to an individual bond pad 14.
This traditional embodiment uses a pattern of individual leads, each of which is substantially perpendicular to the row of bond pads 14, and thus substantially perpendicular to the edge of the semiconductor chip 10. When the lead pattern includes relatively large parallel leads 16, each of which is substantially perpendicular to a single row of bond pads 14 on the semiconductor die 10, there is only a small chance of shorting (by direct contact between wires) the bond wires which connect the lead and the bond pads. This is shown in FIG. 1b, which shows a close-up of a series of bond pads 14 on a semiconductor die 10, and the bond wires 18 connected to the bond pads 14. While it is possible to include bonding pads along only one, two, or three sides of the surface of a semiconductor die, it is now usual to include bonding pads at or near the perimeter of all four edges of a semiconductor die surface.
As semiconductor chips have decreased in size, one of the limiting factors has been number of available I/O pathways for a given chip. As lead and leadframe technology has progressed, it has been desirable to maximize the number of individual leads 16 which are available in a leadframe. As shown in FIG. 2a, lead size, and the distance between leads, has decreased over the years. However, in order to effectively increase the I/O pathways, both the lead count and the number of bonding pads must be increased.
In order to maximize the number of individual bonding pads which are available on the surface of semiconductor die, the trend has been to provide a maximum number of individual bonding pads arrayed about the periphery of a semiconductor die in multiple rows. FIG. 2a shows one edge of a semiconductor die having two separate, staggered rows of bonding pads 14, an outer row 12a and an inner row 12b.
Electrical connection between an individual bonding pad 14 of the semiconductor die and an electrically conductive lead 16 is made by wire bonding, in which a thin connecting wire 18 is bonded at one end to a die bonding pad 14 (for die input or output), and at the opposite end to a lead finger of an electrical lead 16.
The pictured leads 16 are said to have a "fixed pitch", that is, each lead 16 is parallel to the other leads in the pattern. Optimally, each lead 16 is substantially perpendicular to the edge of the semiconductor die 10. However, due to manufacturing slippage, the leads may be slightly or somewhat skewed from the die-perpendicular position. However, each lead remains parallel to other leads in the pattern.
A variety of specific lead systems are known to the art. For example, lead systems can be made which are suitable for dual in-line packages (DIP), quad packages, pin grid array (PGA) packages, flat packs, surface mount devices (SMDs), or the like. Lead systems can comprise a matrix of metal traces formed in situ as part of the packaging process. Generally, however, leads are manufactured separately from a semiconductor package, for example as part of a leadframe or lead system, and then integrated into a semiconductor package. The specific embodiment of a lead system will vary widely with the specific semiconductor chip and packaging system employed.
The lead system can be made from a solid piece of conductive material which is etched, stamped, or otherwise processed to form individual leads 16. A lead system can, but need not, include a die attach pad (not shown). In some applications, the lead system is formed on a non-conductive layer (not shown) or uses a non-conductive tape structure, such as a polyimide layer (not shown), to maintain individual leads in position during processing. A variety of appropriate metals are known for use as lead material, and the metal used will depend upon the desired conductive attributes and cost. Copper, gold, nickel, cobalt, zinc, lead, tin, titanium, and iron leads are especially appropriate, as are alloys or coatings made of conductive materials.
FIG. 2b shows an enlarged view of the bonding wires 18 and bonding pads 14 of a fixed-pitched lead system having two rows of bonding pads 14 on the semiconductor die 10.
As the lead count and bonding pad count have increased, it has become necessary to provide structures which maximize the spacing of both leads and bonding pads. One such arrangement is shown in FIG. 3a. The bonding pads 14 are regularly spaced in two separate rows. The outer row 12a is adjacent the perimeter of the die. The inner row 12b is located toward the interior of the semiconductor die 10 from the outer row.
The leads are arranged "radially", that is, the leads which are medial to the pattern of leads are substantially perpendicular to the edge of the semiconductor die; leads which are positioned away from the median progressively vary from the perpendicular.
As shown in FIG. 3a, a typical radial lead arrangement includes individual leads of varying pitch. Substantially perpendicular leads 16a are generally perpendicular to the edge of the die; that is, they have angles ranging from about 0.degree. to 10.degree. from the perpendicular. Moderately radial leads 16b vary between about 10.degree. and 20.degree. from the perpendicular. Radial leads 16c vary between about 20.degree. and 30.degree. from the perpendicular. Extremely radial leads 16d vary from about 30.degree. to 45.degree. or more from the perpendicular.
Bond wires connecting substantially perpendicular leads 16a to the semiconductor die demonstrate minimal shorting problems, as the bond wires are substantially parallel. There is little variation in the center-to-center measurement along the length of adjacent bond wires. The bonding process is relatively simple, as bonding pads are unlikely to be occluded by previously-bonded wires. As the pitch of the leads, and the pitch of the bond wires, increases in angle from the perpendicular, problems increase. Moderately radial leads 16b demonstrate relatively more problems with shorting of adjacent wires during wire bonding or during fill. Bonding pads are more likely to be occluded by the bonding of adjacent wires. Radial leads 16c show substantial problems with shorting, either during the wire bond process or during fill. Bonding pads are likely to be occluded by previously-bonded wires, which leads to failure of the bond, and failure of the packaged die. Extremely radial leads 16d are very likely to short during wire bonding and/or during fill procedures. The wire pattern for extremely radial leads is such that it is very likely that bond wires will occlude the bonding pads of the adjacent bond wires.
The problems with wire bonding from radial leads 16c and from extremely radial 16d leads is shown in magnified view in FIG. 3b. Due to the radial configuration of the leads (not visible in the magnified view) and the regular spacing of the bonding pads 14, bond wires 18b which are bonded to bonding pads of the inner row 12b pass very close to the bond wires 18a which are bonded to bonding pads of the outer row 12a. This increases the odds of shorting between bond wires during bonding processes or during fill processes. In addition (and as especially visible near the top of the magnified view), the bond wire of one lead may substantially occlude the bonding pad site for bonding an adjacent lead.
Problems in connecting radial lead patterns have been noted in the past. One method for avoiding the shorting and/or the occlusion problems is referred to as "high/low" bonding (not shown). This bonding pattern uses bonding wire loops of different heights to help maintain separation between the individual bonding wires. While high/low bonding patterns provide increased separation between bonding wires during the bonding procedure itself, there are significant drawbacks to the system during fill phases, i.e., when a protective coating is applied over the bond wires or when a housing is formed over the bond wires. The "high" bond wires have increased length, and thus are more flexible than the "low" bond wires. During a fill process, the "high" bond wires are easily flexed out of their original position and into a shorting relationship with adjacent wires. When the bond wire length is in the same direction as the flow direction of the fill material, the problem is somewhat reduced. However, this problem is especially notable when high/low bond wires are located 90.degree. from the source of the fill material.
The prior art bonding patterns and systems have not provided an adequate wire bonding when radial leads are present. Specifically, prior art wire bonds tend to short during bonding or fill procedures when closely positioned adjacent wires touch. Additionally, bonding pads may be occluded by bond wires which have already been placed, leading to a failure of the bond process or to shorting between bond wires.